config: qemu: increase SYS_MALLOC_F_LEN

Several configutation options require additional memory before relocation:

* CONSOLE_RECORD
* LOG
* RSA

The current default of 0x400 is too small to encompass them all. Increase
the value of SYS_MALLOC_F_LEN to 0x2000 for ARCH_QEMU.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>